Concrete is a conglomerate of aggregate (such as gravel, sand, and/or crushed stone), water, and hydraulic cement (such as portland cement), as well as other components and additives. Concrete is initially fluid-like when it is first made, enabling it to be poured or placed into shapes. After hardening this property is lost. When concrete is mixed, it takes about twenty-eight percent of the weight of cement as water to fully consume all the cement to produce hydration products. However, it is not possible to attain a fluid mix with this small amount of water, and more water than is needed is added. The additional water simply resides in the pores present in concrete, and is referred to as the pore liquid or pore solution.
When Portland cement is mixed with water to produce concrete, the alkali oxides present in the cement, Na2O and K2O, dissolve. Alkali materials are supplied by the cement, aggregate, additives, and even from the environment in which the hardened concrete exists (such as salts placed on concrete to melt ice). Thus, the pore solution produced becomes highly basic. It is not unusual for this pore solution to attain a pH of 13.3 or higher. Depending on the aggregate used in the concrete, a highly basic pore solution may interact chemically with the aggregate. In particular, some sources of silica in aggregate react with the pore solution. This process is called the alkali-silica reaction (ASR) and may result in formation of a gelatinous substance which may swell and cause damage to the concrete. This swelling can exert pressures greater than the tensile strength of the concrete and cause the concrete to crack. The ASR reaction takes place over a period of months or years.
Although the reaction is referred to as the alkali-silica reaction, it will be appreciated that it is the hydroxyl ions that are essential for this reaction to occur. For example, ASR will not occur if silica-containing aggregates are placed in contact with alkali nitrate solutions with Na or K concentrations comparable to those which result in ASR if those solutions were alkali hydroxides.
In extreme cases, ASR can cause the failure of concrete structures. More commonly, ASR weakens the ability of concrete to withstand other forms of attack. For example, concrete that is cracked due to this process can permit a greater degree of saturation and is therefore much more susceptible to damage as a result of “freeze-thaw” cycles. Similarly, cracks in the surfaces of steel reinforced concrete can compromise the ability of the concrete to keep out salts when subjected to deicers, thus allowing corrosion of the reinforcing steel.
Concrete used for highways and bridges is periodically exposed to deicing, in which a variety of soluble salts are used, such as NaCl, CaCl2 or MgCl2. Deicing of a surface involves depression of the freezing point of water on the surface. It is well understood that the extent of freezing point depression is dependent mainly on the ionic strength of the aqueous solution. Thus, water containing a higher concentration of ions will freeze at a lower temperature than water containing a lower concentration of ions. For pavement applications, the primary consideration with respect to which soluble salt to use is cost. Typically, NaCl and CaCl2 are used as deicing salts for pavements. Because MgCl2 can be less expensive, it also is used in some applications. However, none of these deicing compounds are used on airfield runway concrete because the chlorides accelerate the corrosion of metals resulting in damage to aircraft. Thus, for airfield runway concrete, the use of deicing compounds that do not damage aircraft is of primary importance. Typical airfield runway deicing compounds are potassium acetate, sodium acetate, potassium formate or sodium formate.
It recently has been discovered that the use of potassium acetate as an airfield runway deicer causes ASR in airfield runway concrete. Set concrete, although hardened contains porosity which can be filled with an aqueous solution of solutes, referred to as “pore solution.” The pore solution typically has a pH higher than 13. One of the products of hydration of cement is solid calcium hydroxide, which typically is in contact with concrete pore solution. The mechanism of airfield runway ASR has not been elucidated fully, but it has been shown that the addition of solid calcium hydroxide to a strong (e.g. 6 molar) solution of potassium acetate causes the pH of concrete pore solution to rise as high as 14 or more. Such a pH likely causes ASR in runway concrete containing siliceous aggregate, which normally would be resistant to such attack.
The following reaction is believed to occur, where the acetate anion is denoted as Ac:KAc(aqueous)+Ca(OH)2(solid) or calcium silicate hydrate (solid)→Ca(Ac)2(solid)+KOH(aqueous). (Either or both of Ca(OH)2 or calcium silicate hydrate can be the calcium source.)
It also has been shown that ASR in airfield runway concrete can occur in the apparent absence of the depletion of calcium hydroxide. However, regardless of the mechanistic uncertainty, the overall mechanism for ASR must include the precipitation of the following metal acetate (Ac): Catx+ (Ac)x, where Cat is any cation, and x is greater than unity. Because acetate salts which include monovalent cations, such as sodium (Na+) or potassium (K+), are more soluble than salts which include divalent or higher valence cations, “x” most likely is an integer of 2 and Catx+ most likely is Ca2+, although Mg2+ cannot be excluded as a possibility. Nevertheless, the potassium hydroxide solution that is formed is responsible for producing the high pH, thereby promoting ASR in the concrete pore solution.
The above overall mechanism likely is generic to the application of this class of deicing salts and will occur regardless of whether the monovalent cation is Na+ or K+ or whether the anion is acetate or formate. Hence, other commercial or similar formulations for runway deicing using sodium acetate, sodium formate or potassium formate, or other organic sodium or potassium salts will produce the same phenomenon.
There are a number of strategies which have been used to mitigate or eliminate ASR. One strategy is to reduce the alkali content of the cement as it is being produced. It is common in cement technology to sum the amounts of K2O and Na2O present and express these as a Na2O equivalent. Cements containing less than 0.6 wt % Na2O equivalent are called low alkali. However, merely using a low alkali cement does not ensure that the alkali silica reaction can be avoided. Another common strategy is the intentional addition of a source of reactive silica, which acts to consume the alkali. Such sources are fine powders and are typically silica fume (a high surface area SiO2 formed as a by-product of making ferro-silicon), fly ash (high surface area materials produced in the combustion of coal which contains SiO2), and natural pozzolans (high surface area materials produced which contains SiO2 and which are typically produced by volcanic action).
Another technology involves the addition of high solubility inorganic lithium compounds such as lithium hydroxide (LiOH) or lithium nitrate (LiNO3). The mechanism of action of Li is not entirely resolved, but it appears to stabilize the alkali silica gels which form. These Li-containing gels then appear to provide a low permeability layer over the underlying reactive silicate material.
However, there are economic and other disadvantages with most of the above-described methods. For example, inorganic lithium salts are expensive and their ability to flow into the pore structures of concrete has not been fully established and they have, therefore, not gained much acceptance. The use of mineral admixtures such as silica fume or fly ash requires additional storage silos, and requires additional mixing. Further, silica fume is expensive, and if not properly blended into the concrete can actually cause ASR. Finally, combustion technology is changing to reduce NOx emissions, which in turn makes fly ash less reactive and thus less suitable as an additive to reduce ASR. Beyond this, fly ash and silica fume are not suitable for treatment of existing structures. There remains a need for economic and effective methods of reducing ASR in concrete.